Radiation sensitive potentiometer with high linearity



June 6, 1967 R. L. WAER 3,324,298

RADIATION SENSITIVE POTENTIOMETER WITH HIGH LINEARITY Filed Feb. 1, 1965 |gure 2 INVENTOR ROBERT L. WAER BY a I ATTORNEY United States Patent 3,324,298 RADIATION SENSITIVE POTENTIOMETER WITH HIGH LINEARITY Robert L. Waer, Los Altos, Califi, assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California Filed Feb. 1, 1965, Ser. No. 429,499 4 Claims. (Cl. 250-211) This invention relates to radiation-controlled potentiometers, and particularly to a high linearity potentiometer having low noise and high reliability. By linearity is meant the linear relationship of voltage and distance along the length of the potentiometer.

Typically high linearity potentiometers comprise a wire wound resistive element connected between a pair of input terminals to serve as the proportioning member of the potentiometer. A sliding mechanical Wiper contacts this wire wound resistive element and couples it to an output terminal. One disadvantage of these wire wound potentiometers is that the output signal obtained therefrom is digital in nature since the sliding mechanical wiper jumps from wire to wire as it is moved along the wire wound resistive element. Furthermore, such potentiometers are unreliable because of intermittent contact between the sliding mechanical wiper and the wire wound resistive element due to dirt in the contact area. Other disadvantages of such potentiometers include abrasion and wear of both the wire wound resistive element and the sliding mechanical Wiper resulting in higher noise and lower reliability.

These disadvantages relating to the sliding action of the mechanical wiper on the wire wound resistive element have been eliminated in conventional photo-potentiometers which, consequently, have low noise and high reliability. However, photo-potentiometers generally do not have the high linearity typical of wire wound potentiometers. For example, the linearity of a photo-potentiometer in which a photoconductive element activated by a movable light beam couples an evaporated film proportioning element to an output terminal is limited to a few percent by present evaporated film techniques.-

Accordingly, it is the principal object of this invention to .providea potentiometric device which incorporates the high linearity of wire Wound potentiometers and the low noise and high reliability of photootentiometers.

It is a more general object of this invention to provide a radiation-controlled potentiometer producing an electrical output signal which is proportional with a high degree of accuracy to a mechanical displacement.

In accordance with the illustrated embodiment of this invention there is provided a potentiometer comprising a substrate with a resistive film proportioning element and a conductive film output terminal placed thereon in adjacent strips continuously spaced a finite distance apart. Conductive film input terminals contacting each end of this resistive film proportioning element are also placed on the substrate. A wire wound resistor is collinearly attached to the resistive film proportioning element and forms therewith the proportioning member of the potentiometer. This proportioning member is coupled to the adjacent output terminal along the full length thereof by a photoconductive element, which is activated by optical means such as a movable light beam.

Other and incidental objects of this invention will become apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is a schematic perspective view of one embodiment of this invention; and

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FIGURE 2 is a schematic top view of another embodiment of this invention.

Referring to FIGURE 1, there is shown an exaggerated perspective view of a radiation-controlled potentiometer 1t) and means 12 for operating the potentiometer in response to translational movement along a track. This radiation-controlled potentiometer 10 includes a substrate 14 having a smooth support face 16 and comprising any electrical insulator, such as glass or ceramic. An e1on-' gated, rectangular conductive layer 18 and a pair of conductive tabs 2t) comprising films of silver or other highly conductive metal are evaporated or applied cold with a brush on the smooth support face 16 of substrate 14. Conductive tabs 20 are placed adjacent to the end portions of conductive layer 18 and are spaced a finite distance therefrom. An elongated, rectangular resistive layer 22 is placed with the end portions thereof overlapping the conductive tabs 20 on the smooth support surface 16 of substrate 14 adjacent to, but not contacting, the conductive layer 18. This resistive layer 22 which comprises, for example, an evaporated film of cermet resistance material serves as a proportioning element for the potentiometer 19. The conductive tabs 20 which contact the end portions of resistive layer 22 serve as power input terminals for the potentiometer 1t and the conductive layer 18 serves as a power output terminal therefor. A photoconductive layer 24 having a high dark resistance couples the resistive layer 22, to the power output terminal, or conductive layer 18. This photoconductive layer 24 comprises, for example, cadmium sulphide or cadmium selenide. Photoconductive layer 24 may be applied to the smooth support .surface 16 of substrate 14 by such methods as evaporation in a vacuum or application with a brush of a suitable paintlike composition of sintered material suspended in a binder which will harden to form a solid coating.

When two dissimilar materials such as the resistive layer 22 and the conductive layer 18 are electrically connected and exposed to radiation such as light, a photovoltaic voltage is generated. This photo-voltaic voltage may be eliminated by placing an elongated, rectangular resistive layer 26, which is similar to resistive layer 22, on the smooth support surface 16 of substrate 14 so as to overlap the conductive layer 18 but not contact the adjacent resistive layer 22. Thus, by placing the photoconductive layer 24 on the smooth support surface 16 of substrate 14 overlapping the similar resistive layers 22 and 26, two similar materials are electrically connected when the photoconductive layer 24 is exposed to radia tion.

A precision wire wound resistor 28 is attached to the resistive layer 22 along the full length thereof by a resistive adhesive layer 30 of resistive epoxy. This wire wound resistor 28 has a high linearity compared to the resistive layer 22 and serves as the proportioning member of the potentiometer 10. The resistive adhesive layer 30 and the resistive layer 22 have a low transverse resistance, but have a high lengthwise resistance; they prevent shorting of the wire wound resistor 28. The per unit length resistance of the wire wound resistor 28 is lower than that of the resistive layer 22.

When the radiation-controlled potentiometer 10-is not illuminated, wire woundresistor 28 and resistive layer- 22 are eifectively isolated electrically from conductive layer 18, due to the high dark resistance of photoconductive layer 24. However, when a portion of the photoconductive layer 24 is illuminated, the resistance of the illuminated portion decreases sharply. Throughout the area of that illumination, wire wound resistor 28 and resistive layer 22 are therefore effectively connected directly to conductive layer 18. For most purposes it is desirable to limit the illumination to a narrow zone which is substantially aligned with a line of uniform potential in the resistive layer 22. That potential is then substantially delivered to the output terminal 18, and may be indicated or utilized in a conventional manner by output means 32. In the present embodiment of this invention, the equipotential lines are straight parallel lines at right angles to the longitudinal axis of the resistive layer 22 since the power input terminals are maintained at different potentials as by a battery 34. Accordingly, the illuminated zone is prefer-ably a narrow, rectangular zone 36 extending across the photoconductive layer 24 at right angles to the longitudinal axis thereof.

Such an illuminated zone may be produced, for example, by a light source 38 comprising an irradiated slit and means for projecting the light 40 therefrom to the upper surface of photoconductive layer 24. Light source 38 may be mounted on a carriage 42 for translational movement along a pair of guide rails 44 disposed above and parallel to the longitudinal axis of the photoconductive layer 24. The position of the illuminated zone 36 may then be moved along the photoconductive layer 24 in response to an input or control mechanism 46 which is appropriately coupled to the carriage 42 as indicated at 48.

Referring now to FIGURE 2, there is shown a schematic top view of a portion of the potentiometer 10 shown in FIGURE 1 as modified to provide a lower potentiometer output impedance. The conductive layer 18 is pro vided with conductive fingers 50 which extend laterally therefrom towards, but not contacting, the resistive layer 22. Similar conductive fingers 52 are placed on the sup port surface of substrate 14 and interstitially spaced between the conductive fingers 50 without contacting the conductive layer 18. Resistive layer 22 overlaps and makes electrical contact with the conductive fingers 52 as well as the conductive tabs 20 which serve as power input terminals for the potentiometer 10. Photoconductive layer 24 is placed on the smooth support surface of substrate 14 intermediate the adjacent edges of conductive layer 18 and resistive layer 22 so as to substantially overlap the conductive fingers 50 and 52. Since conductive fingers 50 and 52 are made of similar materials, the photovoltaic voltage is eliminated in a manner similar to that described in connection with FIGURE 1. The wire wound resistor 28 is attached to the resistive layer 22 along the full length thereof by a resistive adhesive layer (not shown).

When the light source 38 shown in FIGURE 1 is moved along the modified potentiometer 10 of FIGURE 2, a digital output signal will be produced as the irradiated zone passes above successive conductive fingers 50 and 52. This digital effect may be eliminated by adapting the light source 38 to irradiate a narrow, rectangular zone which is disposed at an angle relative to the longitudinal axis of the photoconductive layer 24 so as to cut across more than one conductivefinger.

Though in the illustrated embodiments of this invention the potentiometer 10 is assembled in a rectangular configuration, it may be equally well assembled in some other geometeric configuration as required by particular applications. For example, the potentiometer 10 may be readily assembled in a concentric configuration. The means 12 for operating the potentiometer 10 would then more suitably comprise a light source adapted for rotational movement.

I claim:

1. An electrical resistive device comprising:

a resistive layer;

a precision resistor having a higher linearity than said resistive layer, said precision resistor being electrically and collinearly attached to said resistive layer along one dimension thereof so as to increase the linearity of said resistive device;

at least one electrical connection to said resistive layer;

a conductive electrode disposed adjacent to said resistive layer, said conductive electrode extending along said one dimension of said resistive layer and being continuously spaced a finite distance from said resistive layer;

a photoconductor disposed between the adjacent sides of said resistive layer and said conductive electrode, said photoconductor substantially continuously connecting the adjacent sides of said resistive layer and said conductive electrode;

means for irradiating selectively a limited zone of said photoconductor, said zone extending continuously between said resistive layer and said conductive electrode; and

means for moving said irradiated zone along the length of said photoconductor.

2. An electrical potentiometric device comprising:

a substrate;

a resistive layer supported on said substrate;

a precision resistor having a higher linearity than said resistive layer;

resistive means for electrically and collinearly attaching said precision resistor to said resistive layer along the length of said resistive layer so as to increase the linearity of said potentiometric device;

a pair of conductive input electrodes supported on said substrate, said conductive input electrodes being electrically connected to opposite end portions of said resistive layer;

a conductive output electrode supported on said substrate adjacent to said resistive layer, said conductive output electrode extending along the length of said resistive layer and being continuously spaced a finite distance therefrom;

a photoconductor supported on said substrate between the adjacent sides of said resistive layer and said conductive output electrode, said photoconductor substantially continuously connecting the adjacent sides of said resistive layer and said conductive output electrode;

means for irradiating selectively a limited zone of said photoconductor, said zone extending continuously between said resistive layer and said conductive output electrode so as to provide a conductive path between said precision resistor and said conductive output electrode; and

means for moving said irradiated zone along the length of said photoconductor.

3. An electrical potentiometric device as in claim 2 wherein said conductive output electrode comprises:

an output conductor supported on said substrate; and

another resistive layer supported on said substrate between said photoconductor and said output conductor so as to continuously connect said photoconductor to said output conductor, both of said resistive layers being made of similar material so as to impede generation of a photo-voltaic voltage when said zone of said photoconductor is irradiated.

4. An electrical resistive device comprising:

a resistive layer;

a precision resistor having higher linearity than said resistive layer, said precision resistor being electrically and collinearly attached to said resistive layer along the length thereof so as to increase the linearity of said resistive device;

at least one electrical connection to said resistive layer;

a conductive electrode disposed adjacent to said resistive layer, said conductive electrode extending along the length of said resistive layer and being continuously spaced a finite distance therefrom;

a plurality of conductive fingers alternately contacting said resistive layer and said conductive electrode;

a photoconductor disposed between the adjacent sides of said resistive layer and said conductive electrode, said photoconductor overlapping said conductive fingers;

means for irradiating selectively a limited zone of said photoconductor, said zone extending continuously between a conductive finger contacting said resistive layer and another conductive finger contacting said conductive electrode; and

means for moving said irradiated zone along the length of said photoconductor.

References Cited UNITED STATES PATENTS Bacevicz 250-211 Scott 338-126 X De Gier 250211 X Mongrieff-Yeates 250-211 Jones 250-211 10 WALTER STOLWEIN, Primary Examiner. 

2. AN ELECTRICAL POTENTIOMETRIC DEVICE COMPRISING: A SUBSTRATE; A RESISTIVE LAYER SUPPORTED ON SAID SUBSTRATE; A PRECISION RESISTOR HAVING A HIGHER LINEARITY THAN SAID RESISTIVE LAYER; RESISTIVE MEANS FOR ELECTRICALLY AND COLLINEARLY ATTACHING SAID PRECISION RESISTOR TO SAID RESISTIVE LAYER ALONG THE LENGTH OF SAID RESISTIVE LAYER SO AS TO INCREASE THE LINEARITY OF SAID POTENTIOMETRIC DEVICE; A PAIR OF CONDUCTIVE INPUT ELECTRODES SUPPORTED ON SAID SUBSTRATE, SAID CONDUCTIVE INPUT ELECTRODES BEING ELECTRICALLY CONNECTED TO OPPOSITE END PORTIONS OF SAID RESISTIVE LAYER; A CONDUCTIVE OUTPUT ELECTRODE SUPPORTED ON SAID SUBSTRATE ADJACENT TO SAID RESISTIVE LAYER, SAID CONDUCTIVE OUTPUT ELECTRODE EXTENDING ALONG THE LENGTH OF SAID RESISTIVE LAYER AND BEING CONTINUOUSLY SPACED A FINITE DISTANCE THEREFROM; A PHOTOCONDUCTOR SUPPORTED ON SAID SUBSTRATE BETWEEN THE ADJACENT SIDES OF SAID RESISTIVE LAYER AND SAID CONDUCTIVE OUTPUT ELECTRODE, SAID PHOTOCONDUCTOR SUBSTANTIALLY CONTINUOUSLY CONNECTING THE ADJACENT SIDES OF SAID RESISTIVE LAYER AND SAID CONDUCTIVE OUTPUT ELECTRODE; MEANS FOR IRRADIATING SELECTIVELY A LIMITED ZONE OF SAID PHOTOCONDUCTOR, SAID ZONE EXTENDING CONTINUOUSLY BETWEEN SAID RESISTIVE LAYER AND SAID CONDUCTIVE OUTPUT ELECTRODE SO AS TO PROVIDE A CONDUCTIVE PATH BETWEEN SAID PRECISION RESISTOR AND SAID CONDUCTIVE OUTPUT ELECTRODE; AND MEANS FOR MOVING SAID IRRADIATED ZONE ALONG THE LENGTH OF SAID PHOTOCONDUCTOR. 